In the exploitation of subterranean fluids, particularly petroleum, it is generally desirable to determine the flow characteristics from the reservoir into a borehole drilled from the surface into the subterranean, petroleum-containing formation. When there is sufficient bottom-hole pressure, formation fluids flow naturally into the wellbore and upwardly to the surface. Flow characteristics of the reservoir can be simply determined either by gauging at the surface or by lowering a production logging tool containing a propeller-type flow meter therein into the wellbore.
Some difficulty arises, however, when there is insufficient bottom-hole pressure to produce wellbore fluids to the surface. The hydrostatic column of fluid within the wellbore restricts reservoir fluid entry to the formation face or into the wellbore through the perforations. In order to overcome this hydrostatic column and produce fluids from the well, it is well known in the art to provide "artificial lift" of fluids by injecting a gas, preferably nitrogen, into the wellbore at a depth sufficient to artificially lift wellbore fluids to the surface.
One common way of achieving artificial lift utilizing nitrogen injection is described in U.S. Pat. No. 3,722,589. This technique utilizes coiled tubing which is stored as a continuous length of small diameter pipe on a reel located at the surface. The tubing is injected into the wellbore by a well-known coiled tubing operations employing a tubing injector head located at or near the wellhead. Once the remote end of the coiled tubing has reached the proper depth for gas injection, it is a relatively simple matter of pumping the gas through the coiled tubing to produce the desired artificial lift.
Attempts have been made to log the flow within a wellbore in order to determine various reservoir parameters during the production of wellbore fluids by artificial lift utilizing gas injection with coiled tubing. Some difficulties have been noted in interpreting the data received. Although it is unknown what the true source of the inaccuracies or even the extent of the such inaccuracies may be, it appears that the nature of the apparatus used for such logging may be the source. Typically, the logging tool has been mounted on the coiled tubing immediately below the gas injection orifice. It is therefore possible that nitrogen bubbles are being entrained in the wellbore fluid which is passing through the propeller flow meter of the logging tool. Furthermore, hydrodynamic effects resulting from the injection of the gas into the wellbore fluid may be causing swirls, eddies and the like which may also have an adverse effect on the accuracy of the measurement as determined by the flow meter propeller. Finally, due to the size of the pumping equipment commonly employed with coiled tubing, it is necessary to pump relatively large amounts of gas through the apparatus, a condition which may not facilitate the production of the best data in conjunction with a production logging tool attached to the gas injection tool on coiled tubing.